1. Field of the Invention
The present invention relates to a voltage-controlled oscillator, and more particularly, to a quadrature-phase voltage-controlled oscillator(VCO) used in the receivers and transmitters of wideband wired/wireless transmission systems/networks, products for ultra-wideband communication systems, or wireless network adapters.
2. Description of the Prior Art
In recent years, as the wireless communication technology has rapidly developed and the semiconductor technology has been improved, wireless communication products are facing higher standards. As such, RF circuits are required to operate at higher frequencies and larger amplitudes, with low voltages and low power consumptions. FIG. 1 shows a conventional quadrature-phase VCO using series coupling transistors. The structure of such a quadrature-phase VCO has the following disadvantages:                1. The VCO cannot operate at a low voltage.        2. The flicker noise from the coupling transistors may be up-converted to a frequency near the operation frequency.        3. The coupling transistors require larger sizes, which limits the operation frequency.        4. The VCO is of the source degeneration type, and the transconductance may decrease at high frequencies; thus, high frequency operation is rather limited.        
On the other hand, there are some different quadrature-phase VCOs using coupling coils to overcome the aforementioned disadvantages; however, no conventional quadrature-phase VCO using coupling coils can overcome both the disadvantage that the flicker noise from the coupling transistors may be up-converted to a frequency near the operation frequency and the disadvantage of source degeneration at a high frequency.
In view of the above, the present invention provides a voltage-controlled oscillator operating at higher operation frequencies and larger amplitudes but with low voltages and low power consumptions. The present invention is thus industrially applicable.